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August 12th 08, 04:44 PM
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Forecast of High pressure on Tuesday 19th August
On Aug 12, 3:22*pm, Tudor Hughes wrote:
On Aug 12, 11:32*am, Steve Willington wrote: On 12 Aug, 11:19, Steve Willington wrote: On 12 Aug, 09:26, Dawlish wrote: On Aug 12, 8:18*am, Paul Hyett wrote: On Mon, 11 Aug 2008 at 09:55:06, Dawlish wrote in uk.sci.weather : This forecast is still in the balance for next tuesday - but there's something that we haven't seen on any chart, even at T264+, for weeks. A North African plume!! How is that different from a Spanish one? -- Paul Hyett, Cheltenham (change 'invalid83261' to 'blueyonder' to email me) It has it's source in North Africa - as most plumes that reach the UK do. The hot, dry, often Saharan air becomes substantially modified as it crosses Iberia, on its Northward passage to the UK. "Spanish" plume is a misnomer, as regards the actual source of the hot air, but would be accurate if applied to the source of the modification of that air to produce the potential for thunderstorms in the UK. Look at the source of this hot air in this possible example at +264 on the gfs. http://www.wetterzentrale.de/pics/Rtavn2642.png It is then shown to cross Spain and if it did get to the UK (which I think is unlikely) I'm sure the press would call it a "Spanish" plume; but the source of the hot air would not have been Spain. http://www.wetterzentrale.de/pics/Rtavn3122.png Only the modification would be Spanish - and French too! Of course, pedantically, air has no real "source". It just moves from area to area, but this air mass lingered far longer over North Africa than it is likely to linger over over Spain. Paul There are a few things here that need clarifying: Spanish plume is a generic term for an elevated mixed layer (EML) that, in NW Europe, usually has its origins over hot arid surfaces to the south, often elevated. In the UK the most common source regions are Iberia and the Sahara. More often than not they remain separate on their track to the UK during which time they undergo isentropic up- gliding conserving their potential temperatures. EMLs of Iberian origin occur a little more frequently across the UK and France than those from the Sahara (which, interestingly, are far more commonly found at mid-level above the Caribbean). On many occasions both EMLs will be present clearly identifiable in soundings from their respective potential temperatures which will be closely aligned with source region temperatures, approx 35 to 38C for Iberian and 42 to 46C for Saharan (dependent on time of year), the EML of Saharan origin sitting at a higher level than that sourced from Iberia. EMLs occur in many regions throughout the world and do not necessarily require elevated arid regions to form, some forming over regions where there is a large horizontal gradient in soil moisture. In themselves EMLs are not that exciting, why they are exciting in NW Europe during the warmer months of the year is on occasions when they are advected northwards across France they provide a lid allowing the BL air below this to take up latent and sensible heat elevating WBPTs.. The longer an EML sits over France for instance the higher the BL WBPT that can realised and the more energy available for convection if the lid is weakened, due *to increasing baroclinicity ahead of an upper trough, or if under-running occurs. IOW without the EML you would not attain the high BL WBPTs seen, this is one way of identifying the region overlain by an EML. BTW Ac Cast occurs (normally northern edge) when the EML has been lifted enough for its upper reaches to attain its LCL – it does not tell you anything about the airmass and its stability above this level, it could be capped by a further EML (Saharan overlying Iberian) or subsidence. Steve.www.rockbeareweather.co.uk Sorry for any typos found in the above comments - a quick brain dump with no proof reading! Steve.www.rockbeareweather.co.uk-Hide quoted text - - Show quoted text - * * * * * No typos found, and my first job in Bracknell was as a sub- editor, a job that gives one a hawk eye but also encourages pedantry. More importantly, *a nice clarification of the mechanisms associated with Spanish plumes. *Thanks, Steve. Tudor Hughes, Warlingham, Surrey.- Hide quoted text - - Show quoted text - And North African plumes *)) |
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